1. Field of the Invention
The present invention relates to a wavelength-division multiplexed passive optical network. In particular, it relates to a technology for minimizing the optical loss at a wavelength-division multiplexed passive optical network based on wavelength-locked wavelength-division-multiplexed light sources. It improves the transmission quality and increases the transmission distance.
2. Description of the Related Art
A wavelength-division-multiplexed passive optical network includes a central office, subscribers and an optical distribution network. The optical distribution network connects between the subscribers and the central office without any element requiring the electrical power supply. The optical distribution network has an optical fiber cable and a remote distribution node such as wavelength-division multiplexer/demultiplexer. Wavelength-division multiplexed optical signals are transmitted through an optical fiber cable between the central base station and a remote distribution node. The specific wavelength is allocated to each subscriber.
A wavelength-division multiplexed passive optical network requires numbers of light sources having different wavelengths for allocating at least one wavelength to each subscriber. It also requires a means for recovering faults possibly being occurred at an optical path between the central office and a remote distribution node.
FIG. 1 and FIG. 2 are views illustrating the embodiments of a prior wavelength-division multiplexed passive optical network system.
FIG. 1 shows an embodiment of a wavelength-division multiplexed passive optical network using wavelength-locked wavelength-division-multiplexed light sources. This kind of system is presented in prior documents Kwang-Wook Choo, Chang-Hee Lee and Tae-Won Oh, “Wavelength-division-multiplexing passive optical network based on wavelength-locked wavelength-division-multiplexed light sources through injected incoherent light”, being applied for Korean Patent with an application number of 10-2002-0003318).
Referring to FIG. 1, the wavelength-division-multiplexing passive-optical-network includes a first broadband light source (112) and a second broadband light source (111). The first broadband light source (112) supplies an optical signal containing a first band of wavelengths to a first plurality of optical transmitters (101˜103). The second broadband light source (111) supplies an optical signal containing a second band of wavelengths to a second plurality of optical transmitters (119˜121). A fiber (114) is used for bidirectional transmission of optical signals in at least two different wavelength bands.
A optical coupler (113) operating in both the first band of wavelengths and the second band of wavelengths, wherein the first broadband light source (112) and the second broadband light source (111) couple to the fibers through the optical coupler.
Optical multiplexer/demultiplexers (110, 115) multiplex and demultiplex bi-directionally both the optical signal containing the first band of wavelengths and the optical signal containing the second band of wavelengths.
The broadband filters (107˜109, 116˜118) split the first band of wavelengths and the second band of wavelengths signals to different ports. One port of the broadband filters couples to the first optical transmitter and the other port couples to the second optical receiver.
The first optical multiplexer/demultiplexer (110) spectrally slices a first band of wavelengths received from the first broadband light source (112) and demultiplexes a second band of wavelengths received from the second optical multiplexer/demultiplexer (115). Each optical transmitter in the first group of optical transmitters (101–103) receives a discrete spectrally sliced signal in the first band of wavelengths and aligns the operating wavelength of that optical transmitter to the center wavelength of the received spectrally sliced signal
Similarly, the second optical multiplexer/demultiplexer (115) spectrally slices a second band of wavelengths received from the second broadband light source (111) and demultiplexes a first band of wavelengths received from the first optical multiplexer/demultiplexer (110). Each optical transmitter in the second group of optical transmitters (119–121) receives a discrete spectrally sliced signal in the second band of wavelengths and aligns the operating wavelength of that optical transmitter to the center wavelength of the received spectrally sliced signal.
The optical transmitters (101˜103, 119˜121) are Fabry Perot laser diodes or reflective semiconductor amplifiers.
In the embodiment of a wavelength-division multiplexed passive optical network described above, a optical coupler (113) is used for coupling the first broadband light source (112) and the second broadband light source (111) to the fibers. Moreover, the down-stream signals from the central office to subscribers and the up-stream signals from subscribers to the central office pass the optical coupler (113). The optical coupler (113) is the 2×2 optical splitter, typically 3 dB splitter. The optical splitter causes unnecessary additional optical loss not only at coupling the broadband light source (111–112) to the fibers, but also at the pass of the up-stream signals and the down-stream signals. It increases the optical power of the broadband light source and decreases the transmission length. FIG. 2 shows an embodiment of a network system wherein a fault recovery means, presented in “ITU-T G.983.1 Broadband Optical Access Systems Based on Passive Optical Networks”, a standard proposed by International Telecommunication Union, is applied to the system described in FIG. 1.
If a fault is occurred in an optical cable (114) between a central office and a remote distribution node, all the subscribers can not communicate with each other, and thus a fault recovery function is definitely required.
Referring to FIG. 2, by installing a 1×2 optical spatial switch (206) at the central office, an optical coupler (209) at a remote distribution node (210), the first optical fiber cable (207), and the second optical fiber cable (208), a communication can be carried out through a second optical cable (208) when a trouble is occurred in a first optical cable (207). With this method, however, a optical loss, typically 3 dB, is additionally occurred due to the optical splitter (209) used at a remote distribution node.
As mentioned above, the optical loss, occurred at coupling broadband light sources to the fibers in a wavelength-division multiplexed passive optical network should be minimized. Besides, the optical path loss of signals, occurred when they are being transmitted from an optical transmitter to an optical receiver, makes much effect on transmission quality and expandability. And thus, an appropriate method to minimize and/or compensate the optical loss is required.